The temperature dependence of the flow-stress of heavily-deformed doped tungsten

Abstract

The flow stress of cold-worked, doped tungsten has been determined in the temperature range from 77°K (liquid nitrogen) to about 800°K. The tungsten specimens were fibrous with a <110> texture in the drawing direction. The measured flow stress was attributed to two components. The temperature independent part is due to a long-range dislocation-dislocation interaction and prevails solely at elevated temperatures. The temperature dependent component was analyzed according to a theory of elastic dipoles interacting in a thermally activated process with dislocations. The Gibbs free activation enthalpy, ΔG 0, and the dipole concentration, C g , derived from that analysis are ΔG 0=1.59 eV and C g =800 at. p.p.m., respectively. From these results an activation volume was calculated and compared to the one measured by strain rate changes. The agreement is very good, suggesting that an elastic dipole-dislocation interaction indeed explains the increase of the flow stress with decreasing temperature. The origin of the elastic dipoles cannot be determined unambiguously from this investigation. The elastic dipoles might, however, be due either to impurity atoms in interstitial solution, such as carbon or nitrogen, or to small agglomerates of these impurities. Both possibilities are within the limits of the concentrations found by chemical analysis.